Power transformer having a spring-compressed winding structure

ABSTRACT

Core-form power transformer having a spring loaded winding support structure. The winding is compressed between pressure plates located at the ends of the winding. The compressive force is provided by compressed ring springs which are positioned between structural members of the support structure. The ring springs provide the desired spring constant and damping characteristics for proper winding support during normal and short-circuit conditions.

United States Patent 119 Kaiser n] 3,868,612 1 Feb. 25, 1975 POWERTRANSFORMER HAVING A SPRING-COMPRESSED WINDING STRUCTURE [75] Inventor:

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Mar. 7, 1974 [21] Appl. No.: 448,867

Francis D. Kaiser, Sharon, Pa.

10/1953 Germany ..336/197 3/1944 France 336/197 9/1969 Great Britain336/197 Primary ExaminerThomas J. Kozma l omeniewhenEirm:J.- R. l a

[57] ABSTRACT Core-form power transformer having a spring loaded windingsupport structure. The winding is compressed between pressure plateslocated at the ends of the winding. The compressive force is provided bycompressed ring springs which are positioned between structural membersof the support structure. The ring springs provide the desired springconstant and damping characteristics for proper winding support duringnormal and short-circuit conditions.

1 Claim, 4 Drawing Figures PATEHTED 68251975 3666612 sum 1 or 2 FIG. I

, 1 PowER TRANSFORMER HAVING A SPRING-COMPRESSED WINDING STRUCTUREBACKGROUND OF THE INVENTION l. Field of the Invention This inventionrelates, in general, to electrical inductive apparatus and,specifically, to power transformers .havi'n'g constantly compressedwinding structures.

2. Description of the Prior Art Substantially all large powertransformers have a structural supporting system which is designed torestrain the mechanical movement of the winding structures, especiallyunder the vertical or axial forces generated during short-circuitconditions. Under shortcircuit conditions, the coils of the windingstend to telescope with respect to each other. Therefore, the ends of thewindings must be substantially fixed with respect to each other toprevent distruction of the transformer.

One convenient and useful arrangement forpreventfing telescoping of thewinding coils utilizes pressure plates positioned adjacent to the endsof the winding structure. The pressure plates are firmly held inposition by a rigid support structure. The pressure plates are initiallyforced toward each other during the construction of the transformer tocompress the winding structure. This permits the pressure plates tolimit the movement of thewinding coils immediately uponany tendency totelescope with respect to each other.

The rigid compressing technique used according to thepriorar'thas'some'disadvantages which may be detrimental to theo'pera'tion of the transformer. During normal operation and heatcyclingof the winding str'uc ture, the amount of compressive forces within thewinding structure increase and decrease due to the expansioncharacteristics of the winding structure components and to the limits onthe movement thereof.

After a sufficient length of time, the cycling-enables the windingcomponents to set or acquire a newpermanent dimension which is(generally smaller than the dimension at the time of originalconstruction. Thus, after the transformer has been in operation for aperiod of time, the winding structure tends to become loose and theability of the supporting structure to withstand telescoping forcesunder short-circuit conditions is reduced, sincethe winding coilsacquire a certain amount of inertia before being restrained by thesupporting structure.

To keep the winding structure as tight as possible, for

as long aperiod as possible,*it is generally preferable to compress thewinding structureduringthe manufacturing of the transformer. However,the limits upon the amount of compression which may be safely used isdependent 'upon the winding structure components and the strength of thesupporting structure. In some cases,

the winding insulation may be overstressed mechanithe transformer andwould not provide a practical solution to providing a resilient forcebetween the pressure plates. In addition, the resonant properties oftypical coil springs are detrimental under the influence of theshort-circuit forces exerted on the springs by the coils of thetransformer winding structure. The resonant frequency of any coil springwhich may be used for restraining the pressure plates in a powertransformer is generally close to the frequency at which the force fromthe winding structure is applied to the pressure plates. Thus, withoutsuitable dampening mechanisms, an unstable condition may develop withthe use of normal coil springs and the structural system may failviolently during short-circuitforce conditions.

There have been some attempts to increase the force provided by a coilspring and to dampen the response thereof by effectively providing adashpot inparallel with the spring. With this type of construction,gradual physical changes due to thermal cycling may compress the spring.When the change in physical dimension is extremely rapid, as it would beunder short-circuit stress conditions, the force provided by the springis effectively increased by the action of the dashpot. Thus, a smallamount of telescoping may occur with this type of arrangement. Althoughthis type of arrangement may be feasible for providing thedesired springcharacteristics to support core-form transformer static plates,

tem, and which is suitably dampened to preventoscillation of the forcingmembers due to the frequency of the mechanical stresses produced.

SUMMARY OF THE INVENTION sure plates which are forced toward each otherby a support structure disposed around the magnetic core of thetransformer. The support structure includes a lower end frame which isattached to a lock plate which extends into the upper region of thewinding structure and through a lock plate guide member. Ring springsare positioned between the guide member and a structural brace which iscoupled to the lock plate. The ring springs are initially compressedduring construction of the transformer and constantly exert a force onthe structuralmembers of the support structure to push the pressureplates together, Any expansion of the winding structure or tendency ofthe coils in the winding structure to telescope with respect to eachother must compress the ring springs further. The large spring constantof the ring springs permits a relatively large force to be applied tothe pressure plates and the resistance between adjacent rings of thering springs provides sufficient dampening characteristics to preventoscillation of the ring springs under short-circuit conditions.

BRIEF DESCRIPTION OI THE DRAWING Further advantages and uses of thisinvention will become more apparent when considered in view of thefollowing detailed description and drawing, in which:

. 3 FIG 1 is a sectional view of a core-form power transformerconstructed according to a specific embodiment of this invention,

FIG. 2 is apartial top view of a power transformer constructed accordingto this invention;

:FIG. 3 is a sectional view of a ring spring used in the embodimentshown inFlGS. l and 2; and,

FIG. 4 is a graph representing the load-deflection characteristics ofthe ring spring illustrated in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout-the followingdescription, similar reference characters refer to similar elements ormembers in all of the figures of the drawing.

' Referring nowto the drawing, and to FIG. 1 in particular, there isshown a cross-sectional view of a coreform power transformer constructedaccording to a specific embodiment of this invention. The section istaken generallyalong a plane slightly displaced from the center of a legof the magnetic core of the transformer. The line I -I of FIG. 2indicates the general position of this plane. The transformer coreincludes pressure applied to the winding 16 around the entire peripheryof the winding 16. The pressure plate 20 is positioned on the endframe'22 which is attached to the lock plate 24. A similar arrangementis provided on the other side of the magnetic core 10 by the end frame26 and the lockplate 28. The lock plates 24 and 28 extend upwardlyalongside theleg of the magnetic core 10 into the region above-the upperend ofthe winding structure 16. I I

The endframes 30 and 32 are positioned above the pressure plate 18 andare'connected to the lock plate guide member 34. The guide member 34supports the ring springs36 and 38 which are'positioned between theguide member 34 and the structural brace 40.

The ring springs 36and 38 are initially compressed during theconstruction of the transformer. Therefore, during the operation of thetransformer, the winding pressure applied to the pressure plate -18 bythe end frames and 32 is transmitted through the pressure plate 18around the entire periphery of the winding structure positioned belowthepressure plate 18. Reinforcing ribs 52 positioned on the variousstructural members provide the necessary strength to permit sufficientforce to be applied to the pressure plate 18.

FIG. 3 is a cross-sectional view of. the ring spring 36 shown in FIG. 1illustrating the shape of the rings contained therein. The ring spring36 consist essentially of V a series of rings having conical surfaceswhich are assembled as indicated in FIG.'3. The outside rings 60 have alarger diameter than the inside rings-62. Each of the rings isconstructed ofa suitably tempered material which provides thestress-strain characteristics required. Although the ring spring 36shown in FIG. 3 has larger diameter rings 60 positioned at each ehdthereof, it is within the contemplation of this invention that smallerdiameter rings 62 may be positioned at each end, or that oneendmay havea larger diameter ring 60 and the other end may have a smaller diameterring 62. Although not shown in FIGS. 1 and 3, a suitable guide or sleevemaybe disposed either within or around the ring spring 36 to reduce thepossibility of buckling of the ring spring. I a

A comprehensive technical description of ring springs is con'tainedjin abook by A. Mrwahl, Mechanical Springs, McGraw-Hill 1963, pages.204-2I0.

' Ring springs are available commercially from the Edge water SteelCompany, Pittsburgh, Pa.

Referring again to FIG. 3, as the ring spring 36 is compressed, therings 60 tend to increase in diameter and the material therein issubjected to'tensile stresses. The rings 62 tend to decrease in diameterand are subjected to compressive stresses. At the same time, the

7 thereto. Curve 64 represents the characteristics of the structure 16is subjected to a pressure which tends to i,

move the pressure plates 18 and 20 together. The force providing thispressure is transmitted from the lower ends of the ring springs 36 and'38 through the guide.

member 34 and the end frames 30 and 32, and from the upper ends of thering springs 36 and 38 through the structural brace 40, the lock plates24 and 28, andthe end frames 22 and 26. The initial compression of thering springs 36 and 38 is provided during the construction of thetransformer by applying a downward force 1 to the structural brace 40and inserting suitably sized keys 42 between the structural brace 40 andthe stops 44 which are attached to the lock plates '24 and 28.

FIG. 2 is a partial view of the top of a transformer constructedaccording to this. invention. The guide member 34 contains openings48'and 50 through which the lock plates 28 and 24 extend, respectively.The

spring as it is beingcompressed. Curve 66 represents the characteristicsof the spring as it isbeing recoiled or allowed to return to itsuncompressed state, which is indicated by point 68. When the spring isoriginally compressed, a load L1 is required to deflect the spring adistance D1. As the load is removed from the spring,

the deflection remains constant until the load reaches the value L2. Asthe load decreases below the value L2, the load-deflectioncharacteristics are represented by the curve 66. The hysteresis loop ofthe load-deflection diagram is provided by the frictional forces betweenthe contacting surfaces of the rings 60 and 62.

The particular load-deflection characteristics represented by the ringspring as shown in FIG. 4 are particularly useful for the application ofmaintaining the integrity of large transformer winding structures. Dueto the frictional forces between the rings of the ring spring, the loadrequiredfor a specific amount of deflection is relatively high comparedwith other types of springs. This is advantageous from the standpointthat the physicalsize of thering spring which is necessary to providethe desired load capability has a practical size in relation to othertransformer components. In addition,

ther compressed by the forces occurring during the short-circuitcondition, the large effective spring constant decreases the overalldeflection of the ring spring and prevents excessive movement of thewinding structure. Without excessive compression, the ring springexhibits characteristics close to normal ring springs in regard to theamount of load produced'Thus. the insulation structure of the windingwill not be overstressed by an excessive amount of compression thereon.

The resistance provided by the mating surfaces of the ring springsadditionally helps to dampen any oscillations which may be inherent inthe ring spring components. This reduces the possibility that the forcestending to compress the spring will drive the spring into oscillation.

Since numerous changes may be made in the abovedescribed apparatus, andsince different embodiments of the, invention may be made withoutdeparting from the spirit thereof, it is intended that all of the mattercontained in the foregoing description, or shown in the accompanyingdrawing, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. Electrical inductive apparatus comprising:

a magnetic core having at least one leg;

a winding structure having upper and lower ends an being disposed aroundsaid leg;

a first washer-shaped pressure plate which is positioned adjacent to thelower end of the winding structure;

a first end frame member positioned to push the first pressure plateagainst the winding structure;

a lock plate which is rigidly attached to the first end frame member andwhich extends above the upper end of the winding structure;

a structural brace coupled to said lock plate;

a second washer shaped pressure plate'which is positioned adjacent tothe upper end of the winding

1. Electrical inductive apparatus comprising: a magnetic core having atleast one lEg; a winding structure having upper and lower ends and beingdisposed around said leg; a first washer-shaped pressure plate which ispositioned adjacent to the lower end of the winding structure; a firstend frame member positioned to push the first pressure plate against thewinding structure; a lock plate which is rigidly attached to the firstend frame member and which extends above the upper end of the windingstructure; a structural brace coupled to said lock plate; a secondwasher shaped pressure plate which is positioned adjacent to the upperend of the winding structure; a second end frame member positioned topush the second pressure plate against the winding structure; a lockplate guide member which is attached to the second end frame member forguiding the lock plate; and, at least one ring spring disposed incompressed relationship between the structural brace and the lock plateguide member to force the pressure plates together; said ring springincluding a plurality of rings stacked in axial alignment with eachother, with adjacent rings telescoping with respect to each other whenthe ring spring is compressed.